Simply complete all the story missions on the easiest difficulty to nab this achievement. The training the rookie mission also counts. But you can do this on normal or hard too, as difficulty achievements do stack. Cna be done in either single player or co-op.
To make your life easier, simply go through it on Insane with a buddy playing it on casual and get him to fight everything as they will be scaled to him. There will be a few instances where you have to get your hands dirty though! Can be done in a day and you can also work towards all the followingNOTE: The "ROAD TO RUIN" DLC does not count towards this!